1. Field of the Invention
The present invention relates generally to monitoring systems for use in process manufacturing apparatus and, more specifically, the present invention relates to cavity based ultrasonic resonance monitoring in plastic molding processes and an apparatus for performing the same.
2. Background of the Art
Modern injection molding practice requires the measurement of “plastic” variables such as melt temperature and cavity pressure in order to produce the highest quality polymer parts. While the plastic variables have been shown to be important, sensing these variables has been difficult due to the invasive nature of existing sensing techniques. Three common problems in using ultrasonic sensing techniques have been the high cost of the required equipment, the delicate nature of the equipment, and the level of understanding required to operate the equipment.
The pressure exerted on the mold walls by the melted polymer, or cavity pressure, is currently used in commercial molding systems for control of switch-over between injection phase and packing phase and for Statistical Process Control (“SPC”). These systems utilize a transducer installed in the mold cavity, near a gate, so that the initial pressure rise as the polymer flows past the sensor face is used to detect the position of the polymer melt front in the mold cavity. When the pressure reaches a threshold level, the injection RAM is slowed from high speed injection to low speed packing. In SPC applications, transducers are installed at various locations in the cavity and the shape of the cavity pressure curve is monitored to indicate variation in the molding process. Both of these techniques achieve lower part-to-part variation and higher quality.
The process of injecting a polymer within a mold is typically performed in two stages. The first stage is to rapidly inject the polymer using an injection RAM operating at a first or high speed. The second stage is to slow the injection RAM to a second or low speed to perform packing. The two stages are necessary to rapidly perform the molding sequence while avoiding overpacking or flashing as the mold becomes full.
Various sensing techniques have been developed to measure the temperature of the melted polymer during processing and knowledge of this melt temperature has been used to provide more precise control of the process. Direct measurement of melt temperature provides a distinct advantage since the relationship between the multiple machine variables that affect melt temperature and the actual temperature is quite complex.
The drawback to each of these techniques is primarily the invasive nature of the measurement. Direct cavity pressure measurement requires a hole cut into the mold cavity, which results in a noticeable mark on the molded product. In situations where this is unacceptable, such as automotive lighting lenses, optical disks, et cetera, the sensor cannot be used. The common melt temperature sensing techniques often require a probe extended into the melt and have difficulty in defining the sensed volume reliably. Non-invasive measuring techniques have been developed to correct for these problems. These solutions typically include the use of ultrasonic sensors in both extrusion and injection molding, and as a post manufacture crystallinity sensor. Ultrasound velocity is sensitive to both temperature and pressure in an injection mold cavity. Another solution has utilized ultrasound to monitor the polymer state during extrusion.
While the ultrasound techniques show promise, the equipment typically required in such systems is very expensive, very delicate, and quite complex to operate.
Accordingly, what is needed is a non-invasive monitoring system for use in the molding arts, such as injection or extrusion, that is less expensive than the prior art, less complex, and easier to use than that of the prior art.